Ballast



June 5, 1962 A. E. FEINBERG 3,033,135

BALLAST Filed Jan, 4, 1954 Unite States atent filice 3,038,135 BALLAST Albert E. Feinberg, Chicago, Ill., assignor to Advance Transformer Co., Chicago, 111., a corporation of Illinois Filed Jan. 4, 1954, Ser. No. 401,802 1 Claim. (Cl. 336165) This application is a continuation in part of my application Serial No. 169,940, filed June 23, 1950, now abandoned.

The present invention relates to core structures for shell-type ballasts used with glow discharge lamps, generally of the fluorescent type.

A shell-type ballast is one wherein the magentic structure consists of a generally rectangular laminated shell having a central longitudinally extending laminating core leg, which is referred to as the core, on which the various windings of the ballast are located. Such ballasts have two or more windings spaced lengthwise on the core, and frequently have magnetic shunts bridging the major portion of the space from the core to the shell between adjacent coils to provide controlled leakage flux paths for the coils, the shunts including a very small air gap to prevent saturation of the leakage flux path. By this arrangement a loose magnetic coupling is obtained between the coils.

In shell-type transformers it has heretofore been known to provide an air gap in the magnetic circuit of the primary magnetizing flux, which air gap is also in the circuit of the leakage flux path of one of the secondaries and is outside of the leakage flux path of the primary. In such a ballast, it is of great importance that this gap be of precisely the desired length, which may be of the order of 0.0-1 inch to .04 inch. It is also customary to provide a projection on the core for bridging this air gap. This bridge has definite mechanical functions for holding the core assembly in the shell and for inhibiting vibration noises. This bridge is made very narrow in order to reduce its magnetic eifeot on the reluctance of the air gap insofar as concerns the flux of the magnitudes involved.

Generally the core lamination and the shell lamination are made from a single sheet of metal. The shell lamination is of a hollow rectangular shape, and the core lamination is stamped from the interior of the blank that is to form the shell. Thereafter, a number of shell laminations are assembled together so that they all face in the same direction with respect to the direction in which the punching operation is performed and they are riveted together. In this assembly the burrs that are formed during punching on the respective shell laminations all face in the same direction. A similar number of core laminations are similarly assembled so that the burrs formed on the respective core laminations all face in the same direction in the core assembly, and are riveted together. The cOre laminations and the shell laminations that are assembled together are assembled from the same punching run, although it does not follow that in the assembly the top lamination of the core will be formed from the same sheet that formed the top lamination of the shell. In view of the burrs that are formed on the respective punchings and which appear in the assembly of the shell laminations and in the assembly of the core laminations, it is clear that the core assembly can be reinserted into the shell assembly only by the exertion of considerable pressure. As a result, the shell holds the core under compression and the core holds the shell under tension. Before the magnetic core structure is assembled in the magnetic shell structure the various transformer windings are assembled on the core structure. Thereafter, this core assembly must be pressed into the shell assembly. Since considerable force is necessary to accomplish this result, it is necessary that part of the iron structure of the core must protrude beyond the coils on the core in order to provide a space against which the jaws of the assembly press can bear on the core assembly to force it into the she-ll assembly. To maintain the overall length of the core at a minimum value it must be made of such a size that only a minimum amount thereof projects beyond the coils assembled thereon. Heretofore it has been customary to provide substantial protrusion of the core beyond the coils so that the fingers of the assembling press will have a sufiicient area upon which to bear. In accordance with the principles of the present invention the amount of protrusion of the core beyond the coil at the air gap end of the iron structure is reduced to a minimum and the necessary bearing surface required by the press for pressing the core into the shell is provided by the extension or projection that is to form the bridged air gap. This bridge is therefore made of considerable length. Since the air gap to be bridged is quite short, it becomes necessary to form a socket in the shell for receiving the bridging projection. This provides a substantial area of contact of the bridge with the shell thereby disturbing the required reluctance that was sought to be introduced at the air gap. In order to overcome this difficulty the shell structure is cut away in the region of the bridge to reduce the area of contact between the bridge and the shell while maintaining the bridge in pressure engagement with the shell so that the action of the shell on the bridge maintains the core under compression and the shell under tension.

In pressing the core assembly into the shell assembly the assemblies are arranged so that the burrs on the shell assembly are faced in the direction opposite to the direction in which the burrs face on the core assembly.

The attainment of the above and further objects of the present invention will be apparent from the following specification taken in conjunction with the accompanying drawing forming a part thereof.

In the drawing:

FIG. 1 is a sectional view of a two-lamp fluorescent ballast having a core and shell constructed in accordance with the present invention;

FIG. 2 is an enlarged view of a portion of the core of FIG. 1;

FIG. 3 is an enlarged fragmentary sectional view taken along the line 3-3 of FIG. 2 and looking in the direction of the arrows;

FIG. 4 is an exploded view showing a step in the assembly of the core in the shell; and

FIG. 5 is a circuit diagram showing the application of a ballast of the present invention to a two-lamp fluorescent lighting circuit.

Reference may now be had more particularly to the drawing wherein like reference numerals designate like parts throughout.

At 1 I have shown a ballast the core structure of which comprises a laminated shell 2 of a generally rectangular shape and a centrally located, longitudinally extending laminated core leg 3, hereinafter referred to as the core, that extends between the transverse sides 5 and 6 of the shell and is maintained under longitudinal compression by the transverse sides of the shell. The core and shell laminations are made by stamping from transformer steel that is of a thickness of the order of 0.025 inch. The core laminations and the shell laminations are formed from the same piece of steel, each core lamination being a part of what constituted the interior of the material that formed a shell lamination. The stamped shell laminations are assembled in a stack and riveted together by rivets 10 to form one integral shell. A like number of core laminations are assembled together in a stack and riveted together by rivets 11. When a, is

sheet metal is punched or stamped there is a very slight curvature along the edge of the metal where it has been stamped, forming a distinct burr. In assembling the laminations, all the shell laminations are arranged so that the burrs or curvatures face in the same direction and all the core laminations are similarly assembled.

The assembled shell includes magnetic shunts 15 that extend from the longitudinal sides 1616 of the shell toward the core and almost but not quite touch the core, leaving a short air gap 1717 between each shun-t and the core. In one construction each of these gaps was of an extent of 0.043 inch.

The core assembly 3 includes a projecting tip 20 that is adapted to bear against and fit into a correspondingly shaped groove 21 centered at the inner edge of the transverse side 6 of the shell. This provides a spacing leaving an air gap 24 between the core and the transverse side 6 of the shell. A similar air gap 25 extends between the edge of the core 3 and the shell. The air gaps 24 and 25 provide a controlled magnetic air gap or a controlled reluctance path for the primary magnetizing flux, as will be more fully set forth as this description proceeds. In order to reduce the magnetic path between the transverse sides 6 of the shell and the adjacent end of the core, the area of contact between the tip 20 and the shell is made as small as possible. In order that this may be so in connection with a tip 20 of a considerable extent and a very short air gap 24, the shell is cut out at 27 to reduce the area of contact between the shell and the tip 20.

A set of pre-wound coils 30 is assembled at one end of the core, and a pre-wound coil 31 is assembled on the other end of the core. This constitutes the core assembly. The core assembly 3 is then placed over the shell assembly 1 which rests on a. table or other support and, by means of a press 35, the core is pressed into the shell. The press includes a pair of jaws 36-37 that straddle the coils and bear against the iron structure of the core. To that effect, the jaw 37 bears primarily against the tip 20 of the top lamination of the assembled core structure.

As previously stated, there is a burr on each lamination stamping so that each edge is curved or slopes to one side. As illustrated in FIG. 3, the core larninations are. arranged with their curved or sloping edges 3% all facing in the same direction, which is opposite to the direction of facing of the curved or sloping edges 39 of the shell. As a result of this arrangement, when the core is assembled in the shell there is only a negligible magnetic contact between the end of the tip 2d and the slot 21 that is formed in the transverse side 6 of the shell and into which the tip extends. This con-tact at a series of points between the respective core laminations and the respective shell laminations does not substantially affect the reluctance of the flux path between the core and the side 6 of the shell, but it is sutlicient to maintain the core under compression and the shell under tension and to prevent or substantially reduce vibration of the laminations due to electromagentic effects in the ballast.

Reference may now be had more particularly to FIG. 5 showing the circuit arrangement. In this figure, the coil assembly 30 includes a primary 45, the secondary being the coil 31. The coil assembly 30 also includes three coils 47, 43 and 49 which provide heating currents for the filaments of a pair of rapid start fluorescent type lamps 50 and 51. The secondary 31 is connected into auto step-up transformer relation with the primary and is connected in series with a condenser 53. The lamp 51 is shunted by a condenser 54 that permits voltage to be applied therethrough to the lamp for starting it, after which the lamp 51 starts and then the two lamps operate in series.

In one ballast constructed in accordance with the present invention the coil 45 had 580 turns of wire, the

coils 47, 48 and 49 each had 20 turns of wire and were concentric on the coil 45, the coil 31 had 1235 turns of wire, the condenser 53 was of 2.85 microfarads at 400 volts, and the condenser 54 was of .05 microfarad. The magnetic structure of the ballast was of the construction illustrated in FIG. 1. The length of the shell was 5.182 inches and the width was 2.626 inches. The core was 1.026 inches wide. Each air gap 24 and 25 was 0.026 inch (from the core to the shell). The gap 27 was 0.0968 inch and the tip 29 projected from the edge of the core a distance of .1278 inch. The slot 21 was of a depth of 0.031 inch. The extent to which the edge 60 of the core 3 projected beyond the coil 31 was of the order of 0.0 1 inch or less.

Since it is a characteristic of the so-called rapid start lamps SG-Sl that just enough voltage must be supplied to the circuit so that the lamps will start only when the filaments are properly warmed by the filament current, it is desirable that the maximum peak open circuit voltage applied to the lamps shall be held down to a value insulhcient to ignite the lamps instantly prior to the heating of the filaments and will be sufficient to ignite the lamps after heating of the filaments, and then to operate the lamps. For this type of circuit a ballast having an iron structure such as shown in FIG. 1 gives an excellent ratio of peak to R.M.S. volts. Since this voltage is also the voltage which produces the operating current to the lamp, it is also desirable that the applied voltage be as close to sinusoidal as possible in order that the current flowing through the lamps shall be as close as possible to sinusoidal. By reducing as much as possible the area of contact between the tip 20 and the sides 6 of the shell, the peak effects of the voltage are reduced, thereby approaching more closely a sinusoidal voltage wave shape.

In compliance with the requirements of the patent statutes I have here shown and described a preferred embodiment of my invention. It is, however, to be understood that the invention is not limited to the precise construction here shown, the same being merely illnstrative of the principles of the invention. What I consider new and desire to secure by Letters Patent is:

A transformer having a continuous outer shell of magnetic material in the form of a closed loop, said shell being comprised of one piece laminations, each formed of a pair of spaced longitudinal members and transverse end members integral therewith and joining the opposite ends of said longitudinal members, a separate central core member of magnetic material extending parallel to and spaced from said longitudinal shell members and having ends in press fit engagement with said respective transverse end members of the shell, means providing a bridged air gap between one end face of the central core member and one of said transverse leg members comprising a V-shaped nib of magnetic material converging to the tip thereof and projecting longitudinally from the central portion of said core member, a slot in said transverse end member having a shape complementary to the tip portion of the nib and the defining walls of said slot engaging and snugly receiving only the tip portion of the nib so as to place the core member under longitudinal compression and the shell under tension, respective air gap sections between said one end face of said central core member and said latter transverse leg member on each side of said nib, each air gap section having an inner portion adjacent to said nib formed in part by an indented portion in said transverse leg memher which is several times longer than the adjacent outer portion of each air gap section, said outer shell and core comprised of individual lamination stampings, the stamp ings of the shell and the stampings of the core being respectively arranged with the stamping burrs sloping in the same direction, and the shell and core assemblies being assembled together with their stamping burrs sloping in the opposite direction with the tips of the burrs of one assembly between the burrs of the other assembly.

References Cited in the file of this patent UNITED STATES PATENTS 6 Feinberg June 2 6, 1951 Brooks July 31, 1951 Sola Jan. 15, 1952 Ford Feb. 19, 1952 Dunn Oct. 20, 1953 Strecker Oct. 15, 1957 

